Several techniques and systems have been developed for correcting and stabilizing the spine and for facilitating fusion at various levels of the spine. In one type of system, a bendable rod is disposed longitudinally along the length of the spine or vertebral column. The rod is preferably bent to correspond to the normal curvature of the spine in the particular region being instrumented. For example, the rod can be bent to form a normal kyphotic curvature for the thoracic region of the spine, or a lordotic curvature for the lumbar region. In accordance with such a system, the rod is engaged to various vertebrae along the length of the spinal column by way of a number of fixation elements. A variety of fixation elements can be provided which are configured to engage specific portions of the vertebra. For instance, one such fixation element is a hook that is configured to engage the laminae of the vertebra. Other prevalent fixation elements include spinal screws or bolts, which can be threaded into various portions of vertebral bone.
In one typical procedure utilizing a bendable spinal rod, the rod is situated on opposite sides of the spine or spinous processes. A plurality of fixation elements is attached to a portion of several vertebral bodies. The rods are then affixed to the plurality of fixation elements to apply corrective and stabilizing forces to the spine.
One example of a rod-type spinal fixation system is the TSRH.RTM. Spinal System sold by Danek Medical, Inc. The TSRH.RTM. System includes elongated rods and a variety of hooks, screws and bolts, all configured to create a segmental construct throughout the spine. In one aspect of the TSRH.RTM. System, the spinal rod is connected to the various vertebral fixation elements by way of an eyebolt. In this configuration, the fixation elements are engaged to the spinal rod laterally adjacent to the rod. In another aspect of the TSRH.RTM. System, a variable-angle screw is engaged to the spinal rod by way of an eyebolt. The variable-angle screw allows pivoting of the bone screw in a single plane parallel to the plane of the spinal rod. Details of this variable angle screw can be found in U.S. Pat. No. 5,261,909 to Sutterlin et al., owned by the Assignee of the present invention. One goal achieved by the TSRH.RTM. System is that the surgeon can apply vertebral fixation elements, such as spinal hooks or bone screws, to the spine in appropriate anatomic positions. The TSRH.RTM. System also allows the surgeon to easily engage a bent spinal rod to each of the fixation elements for final tightening.
In recent years, a special material known as "shape-memory alloy" has been used in the construction of various mechanical devices. This type of material is an alloy of known metals, such as copper and zinc, nickel and titanium, silver and cadmium, and others, that are known to exhibit a "shape-memory" in which a particular component formed of a shape-memory alloy (SMA) is capable of reforming to a "memorized" shape at certain temperatures. This shape-memory characteristic occurs when the SMA alloy changes from a martensitic crystal phase to an austenitic crystal phase. In the martensite stage, the SMA is relatively weak and pliable. As the temperature of the SMA component is increased above its transformation temperature range, the SMA transforms to an austenitic stage and the material becomes relatively strong with super-elastic properties. Generally, the strength and super-elastic characteristics of a shape-memory material tend to increase toward the high temperature end of the transformation temperature range and decrease toward the low temperature end. While there are many alloys that exhibit shape-memory characteristics, one of the more common SMAs is an alloy of nickel and titanium. One such well known alloy is Nitinol.RTM. , which has proven to be highly effective for devices to be placed within the human body because its transformation temperature range falls between room temperature and normal human body temperature.
In rod-type spinal fixation systems of the past, set screws typically have been used to fix the location and orientation of hooks or spinal screws along the length of a spinal rod. However, the set screws have been known to have a tendency to back out in in-vivo situations. This could likely cause the device to loosen, thus requiring additional surgery. Moreover, the set screws may strip or gall and their installation can be cumbersome because of the limited amount of room available to manipulate the tools necessary to drive the set screws into their engaged position. There is therefore a need to provide a connecting apparatus, which eliminates reliance on set screws, or other similar devices to affix hooks, bolts or spinal screws to a spinal rod. This need also encompasses a goal of minimizing the profile and bulk of the components used to connect the hooks, bolts or screws to the spinal column. Moreover, it is desirable to reduce the number of components that must be manipulated by the surgeon during a surgical procedure.
While prior attempts have been made to remedy the above-mentioned shortcomings of prior rod-type spinal fixation systems, there is a need remaining in the industry for an improved connecting apparatus that engages and connects two members using shape-memory technology. The present invention meets this need and provides other benefits and advantages in a novel and unobvious manner.